The present invention generally relates to wireless communications receivers, and more particularly relates to techniques for determining the modulation formats and channelization codes used by interfering radio signals.
In today's advanced wireless systems, both the uplink (mobile terminal-to-base station communications) and downlink (base station-to-mobile terminal communications) are subject to various sources of interference, including, for example, intra-cell interference arising from a lack of complete orthogonality between user signals within a wireless system cell, inter-cell interference arising from signals intended for users or originating from users in other cells, and thermal noise. To combat these interference sources, interference cancellation techniques are increasingly being deployed.
One category of interference cancellation techniques is known as decoder interference cancellation (IC) or post-decoder interference cancellation. The general idea behind decoder interference cancellation is that a signal generated using decoder output from a first decoding attempt is subtracted from the input signal before a second decoding attempt. The decoder output from the first decoding attempt could relate to an unwanted signal, for the purpose of cancellation.
FIG. 1 illustrates an example of an interference cancelling receiver 100 that uses output from a decoder to perform interference cancellation. This receiver system is sometimes referred to as a Turbo interference cancellation receiver. In the example figure, a so-called RAKE receiver is shown, which indicates a Wideband Code-Division Multiple Access (W-CDMA) application of decoder interference cancellation.
As can be seen in FIG. 1, the output of the decoder 140 produces log-likelihood ratios (LLR), which are essentially estimated probabilities that the corresponding decoded information bits should be set to one or zero. For interference cancellation purposes, the LLRs are used by soft mapper 150 to generate the symbol values that were most likely to have been transmitted by another node, such as a base station when the receiver is in a wireless terminal. An estimate of the received signal corresponding to these symbol values is produced by signal regenerator 160, which applies the same modulation and scrambling that was removed from the signal by RAKE despreader 110 and demapper 130 in the first decoding iteration. Then, the regenerated signal from signal regenerator 160 is subtracted from the original input signal, using subtracter 105, to produce an interference-reduced signal. At the output of the RAKE despreader 110, the contribution to the despread signal from the original received signal (for a given code) is denoted yk(i) and the contribution from the subtracted signal is denoted as {tilde over (y)}k(i), where i is the symbol index and k the channelization code.
An equalizer, illustrated as a G-RAKE combiner 120 in FIG. 1, applies equalizer weights to the despread signal to reduce the effects of multipath propagation. The equalizer weights, or G-RAKE weights, since the weights are applied after the RAKE, are denoted w. The resulting equalized, despread, symbol samples are then demapped, by demapper 130, to convert them from soft symbols to soft bits.
After demapping from soft symbols to soft bits by demapper 130, decoding is performed by decoder 140, which produces a new (and improved) set of probabilities (LLR) for the transmitted bits. The above procedure could be repeated as many times as desired, subject to limitations on processing power available in the receiver, limitations on latency, etc. Of course, this iterative process might also be terminated when the remaining errors in the decoded bits fall below a target level.
In a High-Speed Downlink Packet Access (HSDPA) link, part of the interference to the desired downlink signal at the wireless terminal arises from High-Speed Physical Downlink Shared Channel (HS-PDSCH) transmissions to other wireless terminals, whether these signals are transmitted from the same transmission points as the desired signal or from neighboring transmission points. These interfering HS-PDSCH transmissions can be at least partly cancelled if the channelization codes and modulation schemes used for these interfering signals are known.
If identifiers for other wireless terminals (user equipment, or UEs, in 3GPP terminology) in the vicinity of a wireless terminal of interest are known, then it is possible to decode scheduling messages sent to these UEs via High-Speed Shared Control Channel messages. These scheduling messages carry information that at least partly defines the channelization codes and modulation schemes to be used in subsequent HS-PDSCH transmissions to that UE. The UE identifier for the targeted UE is used to mask the HS-SCCH messages, which makes it necessary to know the UE identifier to properly decode the HS-SCCH message.
However, even if a receiver knows the UE identifiers for neighboring UEs and is thus able to successfully decode HS-SCCH messages corresponding to interfering HS-PDSCH transmissions, some obstacles remain. One problem is that the HS-SCCH message cannot be interpreted properly unless the receiving unit knows whether the UE targeted by the HS-SCCH has been configured to support 64-QAM operation. This configuration is performed through signaling at higher layers, and it is nearly impossible for a UE other than the one targeted by the configuration message to intercept it. Because the data fields in the HS-SCCH are interpreted differently depending on whether or not 64-QAM operation is configured, a UE eavesdropping on HS-SCCH messages intended for other UEs is still unable to determine the channelization codes and modulation schemes used for HS-PDSCH messages to those UEs from the contents of the HS-SCCH alone. Without knowledge of the channelization codes and modulation schemes used for the interfering HS-PDSCH transmissions, the UE is unable to decode and regenerate the interfering signals as needed to perform interference cancellation.
U.S. Patent Application Publication No. 2010/0260231 describes a method for blind detection of a transport format of a signal, and discloses techniques for reducing the number of transport format hypotheses to be considered in the blind detection. Additional techniques are needed to determine the channelization codes and modulation scheme for interfering HS-PDSCH transmissions.